The typical computer disk drive includes a stack of one or more circular platters and at least one mechanical arm with a read/write head for each surface of each of the platters. The read/write heads read data from or write data to circular tracks on the platters as the platters rotate. The term xe2x80x9ccylinderxe2x80x9d is often used to describe a set of circular tracks, all lying on different disk surfaces at the same radial distance from the centers of the platters.
In accessing a block of addressed data on a particular surface, the mechanical arm positions the corresponding read/write head to the track that stores the data. This movement of the read/write head to the appropriate track is generally called a xe2x80x9cseek.xe2x80x9d The read/write head accesses the targeted block of data when that block rotates to the position of the read/write head. The amount of time required for completing a read or write operation is dominated by the seek-time of the read/write head, the rotation time of the disk, and the data-transfer time. The seek-time is a function of the physical distance traveled by the mechanical arm in moving to the appropriate track.
xe2x80x9cShort-strokingxe2x80x9d is a disk-access technique that involves confining the range-of-movement of the mechanical arm to a small portion of the available cylinders in a disk drive, usually those lying only on the outer tracks of the disks in the drive. Short-stroking improves performance by reducing the distance traveled by the mechanical arm during most disk accesses and thus reducing average seek-time.
Many data-storage systems ensure data integrity and availability by xe2x80x9cmirroringxe2x80x9d the data stored on each disk. Mirroring involves storing a primary copy of data on one disk and a duplicate copy, or xe2x80x9cmirror,xe2x80x9d on another disk drive.
FIG. 1A shows a computing system 100 that uses mirroring to ensure the integrity and availability of data. The system 100 includes one or more central processing units (CPU), or processors 105, that perform certain operations on data in the system. Each processor 105 executes program instructions that are stored, at least temporarily, in one or more volatile or non-volatile memory devices 110, such as a random-access memory (RAM) or read-only memory (ROM) chipset. Much of the data provided to and produced by the processor 105 during these operations is stored on the disk drives 115, 120 in one or more disk subsystems 125. In each subsystem, a hardware controller 130 governs the manner and locations in which data is stored on the disk drives 115, 120. FIG. 1B shows an implementation in which the mirrored disk drives 115, 120 are managed by separate hardware controllers 130, 132.
FIG. 2 shows a common mirroring technique, in which one disk drive 115 serves as the primary disk and another disk drive 120 serves as the mirror disk. The mirror disk 120 stores a mirror copy of all or some portion of the data stored on the primary disk 115. In this example, the disk controller makes essentially the entire surface of each disk available for data storage. As a result, the read/write heads must seek over the entire surface of the primary disk 115 for many read and write operations and must seek over the entire surface of the mirror disk 120 for many write operations. Because all read operations are directed to the primary disk 115, this technique subjects the primary disk to much heavier use than the mirror disk 120.
FIG. 3 shows a mirroring technique that offers better performance than the technique shown in FIG. 1. This technique uses the short-stroking effect to limit the distance traveled by the read/write heads and therefore to reduce the seek-time associated with the disks 115, 120. The disk controller stores data only on the outer portions 135, 140 of the disks 115, 120, leaving the inner portions 145, 150 of the disks unused.
As with the technique shown in FIG. 1, the disk controller stores the primary copy of all data on one disk 115 and the mirror copy on another disk 120. Therefore, this technique also subjects one of the disks to much heavier use than the other.
FIG. 4 shows another mirroring technique that relies on the short-stroking effect. With this technique, the disk controller divides the recording tracks on each disk into distinct groups, or extents, and stores primary and mirror copies of data on alternating extents on each disk. In the example shown here, the disk controller stores the primary copy of one set of data on the outer tracks, or outer extent 155, of one of the disks 115 (xe2x80x9cDisk Axe2x80x9d) and stores the mirror copy of that data on the outer extent 160 of the other disk 120 (xe2x80x9cDisk Bxe2x80x9d). The disk controller then stores the primary copy of another set of data on the inner tracks, or inner extent 165, of Disk B and stores the mirror copy of that data on the inner extent 170 of Disk A. This technique more evenly distributes the workloads of the two disks by placing primary copies of data on both disks, thus ensuring that read operations are directed to both disks.
The short-stroking techniques shown in FIG. 3 and FIG. 4 improve disk-drive performance, as described above, by limiting the distance over which the read/write heads must travel to access data. However, these techniques also leave a large portion of each disk unused, reducing the total storage capacity of the disk drive and thus driving up the cost of disk storage in the system. In all of the techniques of FIGS. 2-4, the primary copy and the mirror copy of each block of data start at the same relative positions, or track offsets, on their respective disks.
A set of storage disks includes a mirrored pair of disks. Each disk in the pair includes a mirror copy of data stored on the other disk in the pair. For each of the disks in the pair, the data forming the mirror copy is physically located between the physical center of the disk and the data that is mirrored on the other disk.